This invention relates to lighting systems for discharge lamps, and pertains more particularly to a lighting system having an inverter and associated means for control of the inverter output frequency for harmlessly and quickly lighting up a discharge lamp as typified by a fluorescent lamp.
It has been known and practiced to incorporate an inverter in discharge lamp lighting systems for higher lighting efficiency and other purposes, as disclosed for example in Japanese Unexamined Patent Publication No. 63-175389. Such conventional lighting systems are alike in having a serial resonant circuit of an inductor and a capacitor connected across an inverter, with the discharge lamp connected in parallel with the capacitor. The discharge lamp has its pair of filamentary electrodes connected in series with the capacitor in order to be preheated.
The magnitude of the current flowing through the LC resonant circuit is frequency dependent, growing to a maximum at a resonance frequency and diminishing in both increasing and decreasing directions from that frequency, because both inductor and capacitor of the resonant circuit inherently possess resistive components. Consequently, the voltage across the capacitor also maximizes at the resonance frequency, which may be in the range of 50-60 kHz, and diminishes in both directions from that frequency. The discharge lamp will therefore be lit up as the inverter output frequency is decremented toward the resonance frequency from a certain higher value, thereby causing a gradual rise in the voltage across the capacitor until an electric discharge starts between the lamp electrodes.
As is well known, an electron radiating substance is coated on the filamentary electrodes of the discharge lamp. In a lighting system including an inverter, the lamp electrodes are preheated as aforesaid, instead of being suddenly subjected to a voltage high enough to initiate an electric discharge therebetween, in order to prevent the electron radiating substance from vaporizing or scattering away from the filaments. Conventionally, the lamp electrodes were preheated for a prescribed period of time by maintaining the voltage across the capacitor at a constant value less than the voltages applied during the subsequent lightup period. The lamp was then lit up by, as aforesaid, decrementing the inverter output frequency and thereby incrementing the voltage across the capacitor until the lamp starts glowing with the commencement of a discharge between the lamp electrodes.
The above conventional practice, briefly holding the inverter output frequency constant for preheating the filamentary lamp electrodes and then decrementing the frequency for lighting up, have proved unsatisfactory for accomplishing the objectives for which it is intended. Experiment has proved that the scattering or vaporizing of the electron radiating substance does take place even by the sudden flow of a preheating current of reduced magnitude through the filaments, making the useful life of the lamp significantly shorter than in the presence of more sophisticated preheating technology.